Multicast relates to the transmission or delivery of a packet, a message, or other information from a single source to a group of destination computers. The bulk of internet traffic is unicast, rather than multicast. Unicast involves data sent to a single destination using a single destination address. Multicast may be implemented in streaming media and internet television applications. In these examples, the implementation of multicast occurs at the IP routing level. Network devices define optimal distribution paths for data sent to multicast destination addresses.
Multicast may be scaled to a large group of recipients without prior knowledge of the identity of individual recipients or how many recipients there are. Multicast uses network infrastructure efficiently by requiring the source to send a packet only once, even if the packet is to be delivered to a large number of recipients. The network devices in the network replicate the packet to reach multiple recipients.
Normally, a network device replicates packets and distributes the replicated packets to egress ports smoothly. However, if one or more ports of the network device become congested, replicated packets are blocked.